Not an easy choice... the 1D series cameras have a much better AF system than the xxD ones (45 point vs 9 point), the AF is faster and more reliable. Other than that you probably wouldn't see a huge improvement, though I imagine that for motorsport faster AF would be a bonus. Other good features are weather sealing, centre point AF at f8, amazing build quality, dual card slots (CF and SD).
The original 1D was only 4mp and often gets bad reviews, however the mkII was 8.2mp and a real step up. In terms of image quality I don't think you'd see any real difference between a mkII and a 30D. The mkIII is 10mp and have the new 14bit processing system which does give even better images, but secondhand they're ~£700 more than a mkII!
As for the D300... from reading reviews it does seem that Nikon are currently leading the way and that there AF system is better than Canon's. Apparently lots of pros have made the change to the dark side, but that comparing the 1D to Nikon's D3 (not the D300). But changing system won't be cheap, and who knows what the new 1D mkIV will be like (rumour has it it will be announced this week).
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